Dave Dugdale from Learning DSLR Video previously carried out a number of different comparisons Nikon D800 and Canon 5D Mark III, now when he chose Canon is time to look for a good lens. He wants select the best 24-70mm f2.8 lens from among three candidates:

At first sight the best for video shooting is the Tamron lens features vibration control (VC):

"I think a lot of people that shoot video are frustrated that Canon didn’t give us a 24-70 with IS. I think Cannon lost an opportunity here and has left the door open for Tamron on this one" - Dave notes.

But the most important factor in this comparison addition to the functionality is sharpness. The principles for assessment as he applied in comparisons, Dave defines as follow:

"If there was an instance where in one test there was one lens sharper in the center but the other was sharper in the corner, I gave the win to the one sharpest in the center. When I had to spend more than 30 seconds trying to figure which one was sharpest I would call it a tie, sometimes it was so close."

Check how these lenses passed the all the benchmarks and which one is best.

Key observations about the sharpness Dave has recorded in several points:

For the most part they all tied in the middle f5.6 to f8 at both wide and tele.

Sigma did really well at f22 and f2.8 at 24mm.

The most glaring however was Sigma at 2.8 at 70mm.

Tamron won only once at f8 at 24mm but it had no glaring issues and way always ever so slightly behind the winners.

The Tamron VC is a lot quieter than my Canon 28-135 IS.

The extensive description of the comparisons you will find the Dava's blog.